So I completed a DOFF mission but it said I could not actually complete because i had too many passengers. So I sold off 5 colonists so I could make room for the 5 from the DOFF mission. Then all was well.

I sold them on the exchange and they were snapped up pretty quickly. Why did someone buy them? What are they using them for?

Also in fleets all three holdings have multiple projects that accept colonists as input. For the starbase and embassy they are projects that take any "civilian" officer and for the mine they take any "common" officer.

Last, KDF side there is a common mission in Klingon territory to sell prisoners to orion slavers for latinum, and it accepts prisoners, refugees and colonists.